In the past, the following two boiler systems have been known: (1) systems where steam is recovered (systems that are fitted with condensate lines), and (2) systems where steam is not recovered (systems that are not fitted with condensate lines). In the case of the former (see FIG. 3), a tank is placed before the boiler, with a deaerator being positioned before said tank. Furthermore, there are such deaeration methods as the chemical injection method, the vacuum method (it can also be performed at elevated temperatures and under increased pressures), the film-type method, etc.
However, in conventional boiler systems, and especially in boiler systems having condensate lines, it has been difficult to supply a boiler with deaerated water having a specified concentration of dissolved oxygen that does not change in the process. The chemical injection method does not insure a complete deaeration (removal of dissolved oxygen), causing the boiler to corrode, while the vacuum method requires large-size equipment or operates under the conditions of elevated temperatures and high pressures even for compact boilers (and is often used with large-size boilers). Film-type deaerators are compartively small when used in boiler systems which do not have condensate lines and can easily supply deaerated water to the boiler, but when a boiler system is fitted with a condensate line for steam recovery, it becomes impossible to render the tank's (the condensate tank's) structure airtight, thereby permitting oxygen which penetrates through the openings to be dissolved again and causing the boiler to corrode.